1. Field of the Invention
The present invention relates to a method and device for inspecting container glass or other transparent or partially transparent containers, and more specifically to a method and device which detect cracks or the like type of defects in such containers.
2. Related Art
In the production of glass used for containers, or in the production of other transparent or also partially transparent containers, quality assurance of the production process is one of the highest priorities. This is especially important in the case of jars for storing food and of glass containers intended for storing pharmaceutical products and while not being limited to aspects of product liability, does find important application in connection therewith.
In this connection, quality assurance has to ensure the absence of cracks in the containers.
The major part of quality assurance methods in current use are still based upon manual visual inspection. However, such inspection can be continuously conducted by people only for a short period of time, since considerable fatigue occurs rapidly usually leading to a drastic rise in the error rate. Inasmuch as this is intolerable for at least the above-mentioned mentioned reasons, frequent and extended breaks are necessary for the personnel involved, resulting in an increased need for manpower and attendant high costs.
Automatic methods for testing for cracks in containers have been previously proposed. These can, on one hand, be differentiated into methods which implement monitoring via the use of a camera for image generation and an accompanying method for evaluating the pictures/images using arrangements such as described in DE 195 38 013 A1 or also by DE 35 32 068 C2, EP 0 456 910 A1, U.S. Pat. Nos. 4,958,223 and 4,701,612. On the other hand, they can be differentiated into methods which use simple optical sensors monitor and rely only on the light intensity, such as described in the DE 35 24 943 A1.
The principal disadvantage of the picture evaluating methods and devices is the enormous amount of computational performance which is required to achieve a practical level of operation. It is imperative to ensure that the testing device does not slow down the output of the machine producing the containers. Consequently either very fast and therefore expensive evaluating computers, as those in highly parallel technology or a number of parallel testing devices are necessary. Both however, involve high costs.
The other approach which uses simple optical sensors which only monitor light intensity, is, in comparison, much easier to implement. Correspondingly, the necessary speed can be attained at a considerably lower cost, as compared with the above mentioned methods. An example of such a testing method and device therefore is described in the DE 35 24 943 A1.
This arrangement makes use of the fact that a light ray striking the posterior surface (interior wall) of a wall of a glass container is nearly reflected in its entirety. If the wall is free of any defect, a reflected light ray leaves the wall again at an angle of reflection equal to that of the angle of incidence. In the case wherein a crack runs through the container wall, the already reflected, exiting light ray is reflected again at the crack (or at an irregularity), and can be received by a suitably arranged detector as an error reflex/indication. Such a device is inexpensive with regard to its components and is furthermore suitable for operating with a very rapid and, in comparison to the picture evaluating methods, less expensive evaluating method.
A principal disadvantage to this, however, is the fact that such devices and the methods necessary for their operation have proven to be not always reliable. It is particularly difficult to adjust them with respect to the material of the container, which changes the angle of reflection. To this effect, blue glass reflects to a much lesser degree than white glass, and it is therefore a problem to reliably adjust a device with respect to the optical properties of the products being tested.
It is the object of the present invention to provide a method/arrangement of the nature disclosed in DE 35 24 943 A1, but which is so improved as to enable testing of transparent or partially transparent containers for cracks and other irregularities (defects) by means of simple optical sensors, wherein only the light intensity is monitored but which ensures adequate reliability and adjustment flexibility with respect to container material.
This problem is solved, according to the invention, by a device and a method for testing for cracks or irregularities (defects) of containers, wherein the device comprises means for moving the container, preferably to rotate, and an optical reception device which converts the light received into an electrical signal which varies in accordance with the intensity of the received light. In addition OT this reception and monitoring means are provided to receive the electrical signal over the time of the movement, and to monitor it with regard to whether the electrical signal exceeds a specific limit value. The device is characterized according to the invention in that the light source is powered by clocked direct current and in that the frequency of the clocked direct current can be adjusted. The method for testing for cracks or irregularities of containers comprises the following method steps:
(i) First, a container is moved, preferably rotated along its longitudinal axis.
(ii) A light ray of clocked constant light (light, which originates from a light source powered by clocked direct current) is directed upon the part of the moving, preferably rotating container.
(iii) A light ray reflected by the container is received by an optical reception device which is positioned in such a way that it lies in or about the focus or the focal level of a light ray which is typically reflected by a crack.
(iv) The optical reception device converts the received light into an electrical signal which changes according to the intensity thereof.
(v) The electrical signal is monitored during the movement of the container to determine if it exceeds a specific limit value.
Optionally the following steps could be subsequently taken:
(vi) If the limiting value is not exceeded during the container movement the examined container is marked as GOOD.
(vii) If the limiting value is exceeded during the movement the respective container tested is marked as POOR.
The clocked direct current referred to is a direct current which is cyclically switched on and off.
The frequency of the clocked direct current used for the operation of the lamp is adjustable with the device according to the invention in such a manner as to allow it to be adapted to the respective optical conditions/characteristics of the containers being inspected.
As a lamp a halogen lamp, possibly a H2 or H4 halogen lamp can be used.
The use of clocked constant light is advisable because measurements with alternating light (light originating from a source powered by alternating current) have proven that this is unsuitable for working of the method according to the invention and in contrast the suitability of clocked constant light was found to be surprisingly good. The frequency for clocking the light is thereby depend ant on the type of material, which may be transparent or semi-transparent material such as glass, from which the containers are made.
It has been shown that blue glass evokes a poorer quality of reflection in comparison to white (viz., water white) glass. The poorer the reflection of the glass (material), the higher the frequency of the light""s clocking should be set. Good results were achieved with H4 halogen lamps at 20 kHz, which have proven to be also especially suited in the detection of cracks in containers made of blue, poorly reflecting glass.
The optical reception device can be embodied as a photo element, preferably as a photodiode.
A preferred embodiment according to the present invention is characterized by a picture reception device which can be used for the reception of a two-dimensional picture, such as a CCD chip.
The optical reception device is preferably mounted on a positioning device which makes it possible to position the optical reception device at different focal levels.
Between the optical reception device and the object to be tested, a lens system can be additionally provided.
By using of a picture reception device for the reception of a two-dimensional picture, such as a CCD chip, an especially clear focusing of the device can be achieved. Here, the picture reception device does not serve, as in the case of using picture evaluating methods for testing for cracks, to produce a complete two-dimensional picture for computer evaluation, but rather for focal adjustment of the system. The optical information received by the picture reception device is routed to a display device, such as a monitor screen. Here, the operator of the device can adjust the system to an optimal state by taking a defective object to be tested, namely one with a crack within the range of the reception area of the picture reception device, and positioning the same in such a way, for example by using the positioning device, that the light rays reflected by the crack lie within the focal level of the picture reception device.
This can be performed with ease when a monitor shows the picture received by the picture reception device thus enabling the operator to receive an optical feedback with respect to the selected adjustment. He/she positions the picture reception device in such a way that the crack can be clearly identified on the monitor. After this it is possible to determine which area of the two-dimensional picture received by the picture reception device, is to be converted with respect to its intensity into an electrical signal for further evaluation. The intensity measured in the selected area is subsequently processed. Thus, the intensity in the relevant area can be evaluated by summing the intensities of individual picture elements and determining a mean value, or the like.
A preferred embodiment of the method for testing for cracks of containers, according to the present invention is further characterized by the fact that the limit value to be monitored in step (v) of the above method is automatically evaluated (updated) starting from a freely (arbitrarily) selected initial value during the working of the method in that a measured maximum value for the reflected light intensity is chosen out of a certain amount of containers each during their movement (which, as noted above, is preferably a rotational movement) and is used as a new limit value for the further working of the method.
In a further embodiment of the method, only such measured maximum values are taken into the automatic evaluation of the limit value, which originate from containers marked as GOOD.
Preferably, the number of containers to be used in the automatic evaluation of the limit value, can be chosen freely.
A certain percentage exceeding of the respective limit value to be monitored is not yet treated as an exceeding of the limit value in a further embodiment for the further course of the method according to the present invention.
Preferably, the size of the percentage exceeding may also be selected freely, here.
A further preferred embodiment of the method according to the present invention here, is characterized by the fact that the signal received in step (iv) is amplified, preferably linearly amplified, before the further working of the method.
A further, particularly preferred embodiment of the method according to the invention is characterized by the fact that before its further processing (monitoring) in step (v), the signal, which is received in step (iv) and possibly already amplified, is first differentiated with respect to time in such a way that it is then no longer the signal or its amplification itself, but its differentiation with respect to time or the differentiation with respect to time of its amplification which is monitored with regard to a limit value. In this manner, a particularly good detection is assured, since cracks in the containers"" material are for the major part, very fine and may occur in an abrupt way. Thus, the electrical signal according to the invention caused by the cracks changes rapidly at the crack edges. However, a rapid change of the signal also effects a particularly strong signal differentiated with respect to time, which is also to be detected quite effectively by means of threshold monitoring.
All in all, the present invention here guarantees a high level of reliability and flexibility in the detection of cracks, whereby it is particularly suitable for optically difficult objects to be tested, and therefore does not need any elaborate and thus cost-intensive picture evaluating methods including the required devices, such as highly parallel computers.